The removal of acid gasses by the amine process is accomplished by a chemical reaction. Physical solvents (Selexol, Sulfinol, Propylene Carbonate) remove acid gases by straight absorption or a combination of absorption and chemical reaction. Amine can be categorized on a chemical basis as being primary (MEA, DGA), secondary (DEA, DIPA) and tertiary (MDEA, TEA) depending on the number of substitutions onto a central nitrogen element. Modern solvents include formulated MDEA (Ucarsol, Gas Spec) and hindered amines (FLEXSORB). This chemical structure influences each amine's properties as a treating solvent and therefore, lend themselves to different applications.